Rui Costa rode to his second stage win of this year's Tour de France Friday as Chris Froome moved ever closer to overall victory in the centenary edition of cycling's most famous race.

Portugal's Costa backed up his triumph in Tuesday's 16th stage into Gap with a similar victory.

The Movistar rider burst out of a chasing group to catch long time leader Pierre Rolland of France (Europcar) on the final big climb of the Col de la Croix Fry and finished clear in Le Grand-Bornand.

German veteran Andreas Kloden (RadioShack) was 47 seconds adrift.

It was also a familiar situation among the leading contenders, with Britain's Froome protecting his five minutes 11 second advantage over Alberto Contador of Spain.

Contador's Saxo-Tinkoff team again launched numerous attacks, but Froome, who had shown signs of weakness on the previous day's finish up L'Alpe d'Huez, easily countered the moves.

Froome, all set to succeed fellow Briton and Sky teammate Bradley Wiggins as Tour de France winner, was again grateful for the support of his loyal lieutenant, Richie Porte of Australia, in a dramatically whittled down main bunch.

"I felt pretty tired after yesterday to be honest but I'm really relieved to have today behind us," he told the official Tour de France website.

"This is an incredible position for me to be in. I mean, to be here one day away from Paris with a five minute advantage on the second place on GC is really a good position to be in but, having said that, I don't want to be too complacent at all."

Saturday will see the Kenyan-born Froome tackle his final challenge in the Alps over 125 km from Annecy to the summit of Annecy-Semnoz.

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Contador is likely to come under pressure from Colombian Nairo Quintana (Movistar), who is third overall at five minutes 32 seconds adrift, but has been showing fine late form in the mountains.

Meanwhile the specter of drugs cast a shadow on proceedings as former Giro d'Italia winner Damiano Cunego was among 28 people connected to the Lampre team to be charged with doping-related offences, by prosecutors in Padua.

Cunego, who is riding this year's Tour de France, but languishes in 56th position, one hour 42 minutes behind Froome, is the biggest name caught up in the potential scandal.

Former world champion Alessandro Ballan is also implicated with a clutch of officials and team doctors.

They are accused of a large scale doping operation, involving the blood-boosting EPO, anabolic steroids, human growth hormone and testosterone.

Lampre mounted a stout defense of its riders and team on its official website, saying they would be able to mount a strong defense when the case is heard in December.

In a statement, the team expressed its "full confidence in team members involved in this affair."